network layer4-1 network layer chapter goals: understand principles behind network layer services:...

Network Layer 4-1

Network Layer

Chapter goals: understand principles behind network

layer services:o routing (path selection)o dealing with scaleo how a router workso advanced topics: IPv6, mobility

instantiation and implementation in the Internet

Network Layer 4-2

Network Layer

4. 1 Introduction 4.2 Virtual circuit

and datagram networks

4.3 What’s inside a router

4.4 IP: Internet Protocolo Datagram formato IPv4 addressingo ICMPo IPv6

4.5 Routing algorithmso Link stateo Distance Vectoro Hierarchical routing

4.6 Routing in the Internet

o RIPo OSPFo BGP

4.7 Broadcast and multicast routing

Network Layer 4-3

Network layer transport segment from sending to receiving

host on sending side encapsulates segments into

datagrams on rcving side, delivers segments to

transport layer network layer protocols in every host, router Router examines header fields in all IP

datagrams passing through it

networkdata linkphysical








applicationtransportnetworkdata linkphysical


Network Layer 4-4

Key Network-Layer Functions

forwarding: move packets from router’s input to appropriate router output

routing: determine route taken by packets from source to dest.

o Routing algorithms

analogy:

routing: process of planning trip from source to dest

forwarding: process of getting through single interchange

Network Layer 4-5

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-6

Connection setup

3rd important function in some network architectures:o ATM, frame relay, X.25

Before datagrams flow, two hosts and intervening routers establish virtual connectiono Routers get involved

Network and transport layer connection service:o Network: between two hostso Transport: between two processes

Network Layer 4-7

Network service model

Q: What service model for “channel” transporting datagrams from sender to rcvr?

Example services for individual datagrams:

guaranteed delivery Guaranteed delivery

with less than 40 msec delay

Example services for a flow of datagrams:

In-order datagram delivery

Guaranteed minimum bandwidth to flow

Restrictions on changes in inter-packet spacing

Network Layer 4-8

Network layer service models:

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

Network Layer 4-9

Network Layer







o RIPo OSPFo BGP


Network Layer 4-10

Network layer connection and connection-less service Datagram network provides network-

layer connectionless service VC network provides network-layer

connection service Analogous to the transport-layer

services, but:o Service: host-to-hosto No choice: network provides one or the

othero Implementation: in the core

Network Layer 4-11

Virtual circuits

call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host

address) every router on source-dest path maintains “state” for

each passing connection link, router resources (bandwidth, buffers) may be

allocated to VC

“source-to-dest path behaves much like telephone circuit”o performance-wiseo network actions along source-to-dest path

Network Layer 4-12

VC implementation

A VC consists of:1. Path from source to destination2. VC numbers, one number for each link along path3. Entries in forwarding tables in routers along path

Packet belonging to VC carries a VC number. VC number must be changed on each link.

o New VC number comes from forwarding table

Network Layer 4-13

Forwarding table

12 22 32

1 32

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 2 222 63 1 18 3 7 2 171 97 3 87… … … …

Forwarding table innorthwest router:

Routers maintain connection state information!

Network Layer 4-14

Virtual circuits: signaling protocols

used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet



1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data

Network Layer 4-15

Datagram networks no call setup at network layer routers: no state about end-to-end connections

o no network-level concept of “connection” packets forwarded using destination host address

o packets between same source-dest pair may take different paths



1. Send data 2. Receive data

Network Layer 4-16

Forwarding table

Destination Address Range Link Interface

11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111

otherwise 3

4 billion possible entries

Network Layer 4-17

Longest prefix matching

Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?

Network Layer 4-18

Datagram or VC network: why?

Internet data exchange among

computerso “elastic” service, no strict

timing req. “smart” end systems

(computers)o can adapt, perform

control, error recoveryo simple inside network,

complexity at “edge” many link types

o different characteristicso uniform service difficult

ATM evolved from telephony human conversation:

o strict timing, reliability requirements

o need for guaranteed service

“dumb” end systemso telephoneso complexity inside

network

Network Layer 4-19

Network Layer







o RIPo OSPFo BGP


Network Layer 4-20

Router Architecture Overview

Two key router functions:

run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link

Network Layer 4-21

Input Port Functions

Decentralized switching: given datagram dest., lookup output

port using forwarding table in input port memory

goal: complete input port processing at ‘line speed’

queuing: if datagrams arrive faster than forwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernetsee chapter 5

Network Layer 4-22

Three types of switching fabrics

Network Layer 4-23

Switching Via MemoryFirst generation routers: traditional computers with switching under direct control of CPUpacket copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram)

InputPort

OutputPort

Memory

System Bus

Network Layer 4-24

Switching Via a Bus

datagram from input port memory

to output port memory via a shared bus

bus contention: switching speed limited by bus bandwidth

1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone)

Network Layer 4-25

Switching Via An Interconnection Network

overcome bus bandwidth limitations Banyan networks, other interconnection nets

initially developed to connect processors in multiprocessor

Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.

Cisco 12000: switches Gbps through the interconnection network

Network Layer 4-26

Output Ports

Buffering required when datagrams arrive from fabric faster than the transmission rate

Scheduling discipline chooses among queued datagrams for transmission

Network Layer 4-27

Output port queueing

buffering when arrival rate via switch exceeds output line speed

queueing (delay) and loss due to output port buffer overflow!

Network Layer 4-28

Input Port Queuing

Fabric slower than input ports combined -> queueing may occur at input queues

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

queueing delay and loss due to input buffer overflow!

Network Layer 4-29

Network Layer







o RIPo OSPFo BGP


Network Layer 4-30

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer 4-31

Network Layer







o RIPo OSPFo BGP


Network Layer 4-32

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

Internet checksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes + app

layer overhead

Network Layer 4-33

IP Fragmentation & Reassembly network links have MTU

(max.transfer size) - largest possible link-level frame.

o different link types, different MTUs

large IP datagram divided (“fragmented”) within net

o one datagram becomes several datagrams

o “reassembled” only at final destination

o IP header bits used to identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Network Layer 4-34

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example 4000 byte

datagram MTU = 1500 bytes

1480 bytes in data field

offset =1480/8

Network Layer 4-35

Network Layer







o RIPo OSPFo BGP


Network Layer 4-36

IP Addressing: introduction IP address: 32-bit

identifier for host, router interface

interface: connection between host/router and physical link

o router’s typically have multiple interfaces

o host may have multiple interfaces

o IP addresses associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

Network Layer 4-37

Subnets IP address:

o subnet part (high order bits)

o host part (low order bits)

What’s a subnet ?o device interfaces

with same subnet part of IP address

o can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

LAN

Network Layer 4-38

Subnets 223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe To determine the

subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. Subnet mask: /24

Network Layer 4-39

SubnetsHow many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

Network Layer 4-40

IP addressing: CIDR

CIDR: Classless InterDomain Routingo subnet portion of address of arbitrary lengtho address format: a.b.c.d/x, where x is # bits in

subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Network Layer 4-41

IP addresses: how to get one?

Q: How does host get IP address?

hard-coded by system admin in a fileo Wintel: control-panel->network->configuration->tcp/ip-

>propertieso UNIX: /etc/rc.config

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as servero “plug-and-play”

(more when we discuss link layer)

Network Layer 4-42

IP addresses: how to get one?

Q: How does network get subnet part of IP addr?A: gets allocated portion of its provider ISP’s

address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

Network Layer 4-43

Hierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routing information:

Network Layer 4-44

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

Network Layer 4-45

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numberso allocates addresseso manages DNSo assigns domain names, resolves disputes

Network Layer 4-46

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

Network Layer 4-47


Motivation: local network uses just one IP address as far as outside word is concerned:o no need to be allocated range of addresses from ISP: - just

one IP address is used for all deviceso can change addresses of devices in local network without

notifying outside worldo can change ISP without changing addresses of devices in

local networko devices inside local net not explicitly addressable, visible

by outside world (a security plus).

Network Layer 4-48


Implementation: NAT router must:

o outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)

. . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr.

o remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair

o incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

Network Layer 4-49


10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345

4

S: 138.76.29.7, 5001D: 128.119.40.186, 80

2

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001

3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

Network Layer 4-50


16-bit port-number field: o 60,000 simultaneous connections with a single LAN-

side address!

NAT is controversial:o routers should only process up to layer 3o violates end-to-end argument

• NAT possibility must be taken into account by app designers, eg, P2P applications

o address shortage should instead be solved by IPv6

Network Layer 4-51

Network Layer







o RIPo OSPFo BGP


Network Layer 4-52

ICMP: Internet Control Message Protocol

used by hosts & routers to communicate network-level information

o error reporting: unreachable host, network, port, protocol

o echo request/reply (used by ping)

network-layer “above” IP:o ICMP msgs carried in IP

datagrams ICMP message: type, code

plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

Network Layer 4-53

Traceroute and ICMP

Source sends series of UDP segments to dest

o First has TTL =1o Second has TTL=2, etc.o Unlikely port number

When nth datagram arrives to nth router:

o Router discards datagram

o And sends to source an ICMP message (type 11, code 0)

o Message includes name of router& IP address

When ICMP message arrives, source calculates RTT

Traceroute does this 3 times

Stopping criterion UDP segment eventually

arrives at destination host

Destination returns ICMP “host unreachable” packet (type 3, code 3)

When source gets this ICMP, stops.

Network Layer 4-54

Network Layer







o RIPo OSPFo BGP


Network Layer 4-55

IPv6 Initial motivation: 32-bit address space

soon to be completely allocated. Additional motivation:

o header format helps speed processing/forwarding

o header changes to facilitate QoS IPv6 datagram format: o fixed-length 40 byte headero no fragmentation allowed

Network Layer 4-56

IPv6 Header (Cont)Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow.” (concept of“flow” not well defined).Next header: identify upper layer protocol for data

Network Layer 4-57

Other Changes from IPv4

Checksum: removed entirely to reduce processing time at each hop

Options: allowed, but outside of header, indicated by “Next Header” field

ICMPv6: new version of ICMPo additional message types, e.g. “Packet Too

Big”o multicast group management functions

Network Layer 4-58

Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneouso no “flag days”o How will the network operate with mixed IPv4

and IPv6 routers? Tunneling: IPv6 carried as payload in IPv4

datagram among IPv4 routers

Network Layer 4-59

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6

B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4

Network Layer 4-60

Network Layer







o RIPo OSPFo BGP


Network Layer 4-61

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

Network Layer 4-62

u

yx

wv

z2

2

13

1

1

2

53

5

Graph: G = (N,E)

N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Graph abstraction

Remark: Graph abstraction is useful in other network contexts

Example: P2P, where N is set of peers and E is set of TCP connections

Network Layer 4-63

Graph abstraction: costs

u

yx

wv

z2

2

13

1

1

2

53

5 • c(x,x’) = cost of link (x,x’)

- e.g., c(w,z) = 5

• cost could always be 1, or inversely related to bandwidth,or inversely related to congestion

Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)

Question: What’s the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path

Network Layer 4-64

Routing Algorithm classification

Global or decentralized information?

Global: all routers have complete

topology, link cost info “link state” algorithmsDecentralized: router knows physically-

connected neighbors, link costs to neighbors

iterative process of computation, exchange of info with neighbors

“distance vector” algorithms

Static or dynamic?Static: routes change slowly

over timeDynamic: routes change more

quicklyo periodic updateo in response to link

cost changes

Network Layer 4-65

Network Layer







o RIPo OSPFo BGP


Network Layer 4-66

A Link-State Routing Algorithm

Dijkstra’s algorithm net topology, link costs

known to all nodeso accomplished via “link

state broadcast” o all nodes have same

info computes least cost paths

from one node (‘source”) to all other nodeso gives forwarding table

for that node iterative: after k

iterations, know least cost path to k dest.’s

Notation: c(x,y): link cost from

node x to y; = ∞ if not direct neighbors

D(v): current value of cost of path from source to dest. v

p(v): predecessor node along path from source to v

N': set of nodes whose least cost path definitively known

Network Layer 4-67

Dijsktra’s Algorithm

1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'

Network Layer 4-68

Dijkstra’s algorithm: example

Step012345

N'u

uxuxy

uxyvuxyvw

uxyvwz

D(v),p(v)2,u2,u2,u

D(w),p(w)5,u4,x3,y3,y

D(x),p(x)1,u

D(y),p(y)∞

2,x

D(z),p(z)∞ ∞

4,y4,y4,y

u

yx

wv

z2

2

13

1

1

2

53

5

Network Layer 4-69

Dijkstra’s algorithm, discussion

Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn)

Oscillations possible: e.g., link cost = amount of carried traffic

A

D

C

B1 1+e

e0

e

1 1

0 0

A

D

C

B2+e 0

001+e1

A

D

C

B0 2+e

1+e10 0

A

D

C

B2+e 0

e01+e1

initially… recompute

routing… recompute … recompute

Network Layer 4-70

Network Layer







o RIPo OSPFo BGP


Network Layer 4-71

Distance Vector Algorithm (1)

Bellman-Ford Equation (dynamic programming)Definedx(y) := cost of least-cost path from x to y

Then

dx(y) = min {c(x,v) + dv(y) }

where min is taken over all neighbors of x

Network Layer 4-72

Bellman-Ford example (2)

u

yx

wv

z2

2

13

1

1

2

53

5Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

du(z) = min { c(u,v) + dv(z), c(u,x) + dx(z), c(u,w) + dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4

Node that achieved minimum is nexthop in shortest path ➜ forwarding table

B-F equation says:

Network Layer 4-73


Dx(y) = estimate of least cost from x to y Distance vector: Dx = [Dx(y): y in N ] Node x knows cost to each neighbor v: c(x,v) Node x maintains Dx = [Dx(y): y in N ] Node x also maintains its neighbors’ distance

vectorso For each neighbor v, x maintains

Dv = [Dv(y): y in N ]

Network Layer 4-74

Distance vector algorithm (4)

Basic idea: Each node periodically sends its own distance

vector estimate to neighbors When a node x receives new DV estimate

from neighbor, it updates its own DV using B-F equation:

Dx(y) ← minv{c(x,v) + Dv(y)} for each node y in N

Under minor, natural conditions, the estimate Dx(y) converges to the actual least cost dx(y)

Network Layer 4-75


Iterative, asynchronous: each local iteration caused by:

local link cost change DV update message from

neighbor

Distributed: each node notifies

neighbors only when its DV changes

o neighbors then notify their neighbors if necessary

wait for (change in local link cost of msg from neighbor)

recompute estimates

if DV to any dest has

changed, notify neighbors

Each node:

Network Layer 4-76

x y z

xyz

0 2 7

∞ ∞ ∞∞ ∞ ∞

from

cost to

from

from

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 3

from

cost to

x y z

xyz

∞ ∞

∞ ∞ ∞

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

0 2 3

from

cost to

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

∞ ∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)} = min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)} = min{2+1 , 7+0} = 3

Network Layer 4-77

Distance Vector: link cost changes

Link cost changes: node detects local link cost

change updates routing info, recalculates

distance vector if DV changes, notify neighbors

“goodnews travelsfast”

x z14

50

y1

At time t0, y detects the link-cost change, updates its DV, and informs its neighbors.

At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV.

At time t2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z.

Network Layer 4-78

Distance Vector: link cost changes

Link cost changes: good news travels fast bad news travels slow - “count to

infinity” problem! 44 iterations before algorithm

stabilizes: see text

Poisoned reverse: If Z routes through Y to get to X :

o Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)

will this completely solve count to infinity problem?

x z14

50

y60

Network Layer 4-79

Comparison of LS and DV algorithms

Message complexity LS: with n nodes, E links,

O(nE) msgs sent DV: exchange between

neighbors onlyo convergence time varies

Speed of Convergence LS: O(n2) algorithm requires

O(nE) msgso may have oscillations

DV: convergence time varieso may be routing loopso count-to-infinity problem

Robustness: what happens if router malfunctions?

LS: o node can advertise

incorrect link costo each node computes only

its own tableDV:

o DV node can advertise incorrect path cost

o each node’s table used by others

• error propagate thru network

Network Layer 4-80

Network Layer







o RIPo OSPFo BGP


Network Layer 4-81

Hierarchical Routing

scale: with 200 million destinations:

can’t store all dest’s in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

each network admin may want to control routing in its own network

Our routing study thus far - idealization all routers identical network “flat”… not true in practice

Network Layer 4-82

Hierarchical Routing

aggregate routers into regions, “autonomous systems” (AS)

routers in same AS run same routing protocol

o “intra-AS” routing protocol

o routers in different AS can run different intra-AS routing protocol

Gateway router Direct link to router

in another AS

Network Layer 4-83

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

Forwarding table is configured by both intra- and inter-AS routing algorithmo Intra-AS sets entries

for internal destso Inter-AS & Intra-As

sets entries for external dests

Network Layer 4-84

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

Inter-AS tasks Suppose router in AS1

receives datagram for which dest is outside of AS1

o Router should forward packet towards one of the gateway routers, but which one?

AS1 needs:1. to learn which dests

are reachable through AS2 and which through AS3

2. to propagate this reachability info to all routers in AS1

Job of inter-AS routing!

Network Layer 4-85

Example: Setting forwarding table in router 1d

Suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 (gateway 1c) but not from AS2.

Inter-AS protocol propagates reachability info to all internal routers.

Router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c.

Puts in forwarding table entry (x,I).

Network Layer 4-86

Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routing:Choose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface I that leads

to least-cost gateway. Enter (x,I) in

forwarding table

Example: Choosing among multiple ASes

Now suppose AS1 learns from the inter-AS protocol that subnet x is reachable from AS3 and from AS2.

To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x.

This is also the job on inter-AS routing protocol! Hot potato routing: send packet towards closest of

two routers.

Network Layer 4-87

Network Layer







o RIPo OSPFo BGP


Network Layer 4-88

Intra-AS Routing

Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols:

o RIP: Routing Information Protocol

o OSPF: Open Shortest Path First

o IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

Network Layer 4-89

Network Layer







o RIPo OSPFo BGP


Network Layer 4-90

RIP ( Routing Information Protocol)

Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops)

DC

BA

u v

w

x

yz

destination hops u 1 v 2 w 2 x 3 y 3 z 2

Network Layer 4-91

RIP advertisements

Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)

Each advertisement: list of up to 25 destination nets within AS

Network Layer 4-92

RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B 7

x -- 1…. …. ....

w x y

z

A

C

D B

Routing table in D

Network Layer 4-93

RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B A 7 5

x -- 1…. …. ....Routing table in D

w x y

z

A

C

D B

Dest Next hops w - - x - - z C 4 …. … ...

Advertisementfrom A to D

Network Layer 4-94

RIP: Link Failure and Recovery If no advertisement heard after 180 sec -->

neighbor/link declared deado routes via neighbor invalidatedo new advertisements sent to neighborso neighbors in turn send out new advertisements

(if tables changed)o link failure info quickly propagates to entire neto poison reverse used to prevent ping-pong

loops (infinite distance = 16 hops)

Network Layer 4-95

RIP Table processing

RIP routing tables managed by application-level process called route-d (daemon)

advertisements sent in UDP packets, periodically repeated

physical

link

network forwarding (IP) table

Transprt (UDP)

routed

physical

link

network (IP)

Transprt (UDP)

routed

forwardingtable

Network Layer 4-96

Network Layer







o RIPo OSPFo BGP


Network Layer 4-97

OSPF (Open Shortest Path First)

“open”: publicly available Uses Link State algorithm

o LS packet disseminationo Topology map at each nodeo Route computation using Dijkstra’s algorithm

OSPF advertisement carries one entry per neighbor router

Advertisements disseminated to entire AS (via flooding)o Carried in OSPF messages directly over IP (rather than

TCP or UDP

Network Layer 4-98

OSPF “advanced” features (not in RIP)

Security: all OSPF messages authenticated (to prevent malicious intrusion)

Multiple same-cost paths allowed (only one path in RIP)

For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time)

Integrated uni- and multicast support: o Multicast OSPF (MOSPF) uses same topology

data base as OSPF Hierarchical OSPF in large domains.

Network Layer 4-99

Hierarchical OSPF

Network Layer 4-100

Hierarchical OSPF

Two-level hierarchy: local area, backbone.o Link-state advertisements only in area o each nodes has detailed area topology; only

know direction (shortest path) to nets in other areas.

Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

Backbone routers: run OSPF routing limited to backbone.

Boundary routers: connect to other AS’s.

Network Layer 4-101

Network Layer







o RIPo OSPFo BGP


Network Layer 4-102

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto standard

BGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.2. Propagate the reachability information to all routers

internal to the AS.3. Determine “good” routes to subnets based on

reachability information and policy. Allows a subnet to advertise its existence to

rest of the Internet: “I am here”

Network Layer 4-103

BGP basics Pairs of routers (BGP peers) exchange routing info over

semi-permanent TCP conctns: BGP sessions Note that BGP sessions do not correspond to physical links. When AS2 advertises a prefix to AS1, AS2 is promising it

will forward any datagrams destined to that prefix towards the prefix.

o AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

Network Layer 4-104

Distributing reachability info With eBGP session between 3a and 1c, AS3 sends prefix

reachability info to AS1. 1c can then use iBGP do distribute this new prefix reach

info to all routers in AS1 1b can then re-advertise the new reach info to AS2 over

the 1b-to-2a eBGP session When router learns about a new prefix, it creates an

entry for the prefix in its forwarding table.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

Network Layer 4-105

Path attributes & BGP routes

When advertising a prefix, advert includes BGP attributes. o prefix + attributes = “route”

Two important attributes:o AS-PATH: contains the ASs through which the advert

for the prefix passed: AS 67 AS 17 o NEXT-HOP: Indicates the specific internal-AS router

to next-hop AS. (There may be multiple links from current AS to next-hop-AS.)

When gateway router receives route advert, uses import policy to accept/decline.

Network Layer 4-106

BGP route selection

Router may learn about more than 1 route to some prefix. Router must select route.

Elimination rules:1. Local preference value attribute: policy

decision2. Shortest AS-PATH 3. Closest NEXT-HOP router: hot potato

routing4. Additional criteria

Network Layer 4-107

BGP messages

BGP messages exchanged using TCP. BGP messages:

o OPEN: opens TCP connection to peer and authenticates sender

o UPDATE: advertises new path (or withdraws old)

o KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request

o NOTIFICATION: reports errors in previous msg; also used to close connection

Network Layer 4-108

BGP routing policy

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks

o X does not want to route from B via X to Co .. so X will not advertise to B a route to C

Network Layer 4-109

BGP routing policy (2)

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW?

o No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

o B wants to force C to route to w via Ao B wants to route only to/from its customers!

Network Layer 4-110

Why different Intra- and Inter-AS routing ?

Policy: Inter-AS: admin wants control over how its traffic

routed, who routes through its net. Intra-AS: single admin, so no policy decisions

needed

Scale: hierarchical routing saves table size, reduced

update trafficPerformance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance

Network Layer 4-111

Network Layer







o RIPo OSPFo BGP


Network Layer 4-112

R1

Source-duplication versus in-network duplication. (a) source duplication, (b) in-network duplication

R2

R3 R4

(a)

R1

R2

R3 R4

(b)

duplicatecreation/transmissionduplicate

duplicate

Network Layer 4-113

A

Controlled flooding: Reverse path forwarding

B

G

DE

c

F

Network Layer 4-114

Broadcast along a spanning tree

A

B

G

DE

c

F

A

B

G

DE

c

F

(a) Broadcast initiated at A (b) Broadcast initiated at D

Network Layer 4-115

Center-based construction of a spanning tree

A

B

G

DE

c

F1

2

3

4

5

(a) Stepwise construction of spanning tree

A

B

G

DE

c

F

(b) Constructed spanning tree

Multicast Routing: Problem Statement Goal: find a tree (or trees) connecting

routers having local mcast group members o tree: not all paths between routers usedo source-based: different tree from each sender to rcvrso shared-tree: same tree used by all group members

Shared tree Source-based trees

Approaches for building mcast treesApproaches: source-based tree: one tree per source

o shortest path treeso reverse path forwarding

group-shared tree: group uses one treeo minimal spanning (Steiner) o center-based trees

…we will only study the high-level approaches

Shortest Path Tree

mcast forwarding tree: tree of shortest path routes from source to all receiverso Dijkstra’s algorithm

R1

R2

R3

R4

R5

R6 R7

21

6

3 4

5

i

router with attachedgroup member

router with no attachedgroup member

link used for forwarding,i indicates order linkadded by algorithm

LEGENDS: source

Reverse Path Forwarding

if (mcast datagram received on incoming link on shortest path back to center)

then flood datagram onto all outgoing links

else ignore datagram

rely on router’s knowledge of unicast shortest path from it to sender

each router has simple forwarding behavior:

Reverse Path Forwarding: example

• result is a source-specific reverse SPT– may be a bad choice with asymmetric links

R1

R2

R3

R4

R5

R6 R7



datagram will be forwarded

LEGENDS: source

datagram will not be forwarded

Reverse Path Forwarding: pruning forwarding tree contains subtrees with no mcast

group members (Used by DVMRP and PIM-Dense)o no need to forward datagrams down subtreeo “prune” msgs sent upstream by router with

no downstream group members

R1

R2

R3

R4

R5

R6 R7



prune message

LEGENDS: source

links with multicastforwarding

P

P

P

Shared-Tree: Steiner Tree

Steiner Tree: minimum cost tree connecting all routers with attached group members

problem is NP-complete excellent heuristics exists not used in practice:

o computational complexityo information about entire network neededo monolithic: rerun whenever a router needs to

join/leave

Center-based trees

single delivery tree shared by all one router identified as “center” of tree to join:

o edge router sends unicast join-msg addressed to center router

o join-msg “processed” by intermediate routers and forwarded towards center

o join-msg either hits existing tree branch for this center, or arrives at center

o path taken by join-msg becomes new branch of tree for this router

Used by PIM in sparse mode

Center-based trees: an example

Suppose R6 chosen as center:

R1

R2

R3

R4

R5

R6 R7



path order in which join messages generated

LEGEND

21

3

1

Network Layer 4-125

Network Layer: summary

Next stop: the Data

link layer!

What we’ve covered: network layer services routing principles: link state and

distance vector hierarchical routing IP Internet routing protocols RIP, OSPF,

BGP what’s inside a router? IPv6

network layer4-1 network layer chapter goals: understand principles behind network layer services:...

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